Zn-Al-Mg-Si-alloy plated steel product having excellent corrosion resistance and method for preparing the same

ABSTRACT

A Zn—Al—Mg—Si alloy-plated steel material with excellent corrosion resistance, characterized by comprising, in terms of wt %, Al: at least 45% and no greater than 70%, Mg: at least 3% and less than 10%, Si: at least 3% and less than 10%, with the remainder Zn and unavoidable impurities, wherein the Al/Zn ratio is 0.89-2.75 and the plating layer contains a bulky Mg 2 Si phase; also, a Zn—Al—Mg—Si alloy-plated steel material with excellent corrosion resistance, characterized by comprising, in terms of wt %, Al: at least 45% and no greater than 70%, Mg: at least 1% and less than 5%, Si: at least 0.5% and less than 3%, with the remainder Zn and unavoidable impurities, wherein the Al/Zn ratio is 0.89-2.75 and the plating layer contains a scaly Mg 2 Si phase.

TECHNICAL FIELD

The present invention relates to a highly corrosion resistantAl—Zn—Mg—Si alloy-plated steel material and to a process for itsproduction.

BACKGROUND ART

Zn plating of steel surfaces for improved corrosion resistance has beenwidely known in the prior art, and materials with Zn platings arecurrently produced in mass. Zn—Al alloy platings have even been proposedas a means of further improving corrosion resistance. Such an Zn—Alalloy plating is proposed in Japanese Patent No. 617,971. Specifically,there is disclosed an alloy plating comprising Al at 25-75%, Si at 0.5%or more of the Al content and with the remainder consisting ofsubstantially Zn, wherein the Zn—Al alloy obtained exhibits excellentcorrosion resistance as well as satisfactory adhesion to steel sheetsand an attractive outer appearance. Such Zn—Al alloy platings provideespecially excellent corrosion resistance compared to conventional Znplatings.

It is currently the situation, however, that when Zn—Al plated steelsheets fabricated in this manner are subjected to cutting, the exhibitedcorrosion resistance at the cut edges is insufficient. This occursbecause, although corrosion of the steel sheet sections exposed at thecut edges is prevented by the sacrificial rusting effect of the Zn, theZn component is lost from the Zn-segregated sections of the Zn—Al alloyplating layer, thus lowering the corrosion resistance. Also, when theplating layer is further coated with paint or laminated with a plasticfilm, the corrosion product resulting from selective corrosion of Znaccumulates, creating film blisters or so-called edge creep, and thusnotably reducing the product value.

As a means of improving the corrosion resistance of cut edges of paintedZn—Al alloy platings, Japanese Patent No. 1,330,504 discloses an alloyplating containing Mg at 0.01-1.0% in a Zn—Al alloy layer, and althougha slight effect is exhibited, the technique does not provide a thoroughsolution to the problem of edge corrosion. A similar technique isdisclosed in Japanese Examined Patent Publication HEI No. 3-21627, as aplating which comprises 3-20% Mg, 3-15% Si and the remainder Al and Znwith an Al/Zn ratio of 1-1.5, and which is characterized by having astructure with Al-rich dendritic crystals as well as Zn-rich dendriticcrystals and an intermetallic compound phase comprising Mg₂Si, MgZn₂,SiO₂ and Mg₃₂(Al,Zn)₄₉.

The results of experimentation by the present inventors have revealedthat although plated steel sheets employing the platings disclosed inthe prior art sometimes exhibit vastly improved corrosion resistancecompared to Zn—Al plated steel sheets containing no Mg or Si, theworkability of the platings differs depending on the Mg and Si content,and on the proportion and the form and size of the deposited Mg₂Siphase, such that considerable variation is exhibited in terms of thecorrosion resistance. Particularly as concerns the size of the Mg₂Siphase, the observed size also differs depending on the method ofobserving the structure, and especially depending on the sampleembedding angle when observing the cross-sectional composition, and itwas found to be important to carry out measurement of the size by a moreprecise method while controlling the size.

It was also found that if the content of the deposited Mg₂si phase iskept at above a certain value, even outside of the range of thecomposition disclosed in the aforementioned prior art, there exists arange in which the corrosion resistance is vastly improved compared toconventional Zn—Al plated steel sheets.

Another prior art example of controlling the amount of the Mg₂Si phasein the plating phase is found in U.S. Pat. No. 3,026,606, whichdiscloses a technique whereby the Mg₂Si phase in the Al plating phase iscontrolled in a range of 4-25% and the thickness of the alloy phaseproduced at the interface between the plating phase and the base iron isminimized; however, the Mg₂Si phase is not utilized as the means forimproving corrosion resistance.

The present invention provides a highly corrosion resistant Zn—Al—Mg—Sialloy-plated steel sheet having a controlled content of Mg and Si addedto a Zn—Al based plating and a controlled deposition amount anddeposition form of the Mg₂Si phase which exhibits an effect of improvingcorrosion resistance, as well as a process for its production.

DISCLOSURE OF THE INVENTION

As a result of diligent research aimed at solving the problems describedabove, the present inventors have completed the present invention uponfinding that by adding Mg and Si in an appropriate range to Zn—Al alloyand controlling the structure thereof, it is possible to provide analloy plating with not only unpainted corrosion resistance but alsoexceptional edge creep resistance at cut edge sections after painting,which has not been achievable by the prior art.

In other words, the gist of the present invention is as follows. (

1) A Zn—Al—Mg—Si alloy-plated steel material with excellent corrosionresistance, characterized by comprising, in terms of wt %,

Al: at least 45% and no greater than 70%

Mg: at least 3% and less than 10%

Si: at least 3% and less than 10%,

with the remainder Zn and unavoidable impurities, wherein the Al/Znratio is 0.89-2.75 and the plating layer contains a bulky Mg₂Si phase.

(2) A Zn—Al—Mg—Si alloy-plated steel material with excellent corrosionresistance, characterized by comprising, in terms of wt %,

Al: at least 45% and no greater than 70%

Mg: at least 1% and less than 5%

Si: at least 0.5% and less than 3%,

with the remainder Zn and unavoidable impurities, wherein the Al/Znratio is 0.89-2.75 and the plating layer contains a scaly Mg₂Si phase.

(3) A Zn—Al—Mg—Si alloy-plated steel material with excellent corrosionresistance according to (1) or (2) above, characterized by furthercomprising, as the Zn—Al—Mg—Si alloy plating composition, one or morefrom among In: 0.01-1.0%, Sn: 0.1-10.0%, Ca: 0.01-0.5%, Be: 0.01-0.2%,Ti: 0.01-0.2%, Cu: 0.1-1.0%, Ni: 0.01-0.2%, Co: 0.01-0.3%, Cr:0.01-0.2%, Mn: 0.01-0.5%, Fe: 0.01-3.0% and Sr: 0.01-0.5%.

(4) A Zn—Al—Mg—Si alloy-plated steel material with excellent corrosionresistance, characterized in that the bulky Mg₂Si phase of (1) above hasa long diameter mean size of 3-50 μm, the area ratio of particles with along diameter exceeding 100 μm is no more than 10% of the bulky Mg₂Siphase, and the ratio of the short diameter to the long diameter is atleast 0.4, as observed with a 5° inclination polished cross-section.

(5) A Zn—Al—Mg—Si alloy-plated steel material with excellent corrosionresistance, characterized in that the scaly Mg₂Si phase of (2) above hasa long diameter mean size of 3-50 μm, and the ratio of the shortdiameter to the long diameter is less than 0.4, as observed with a 5°inclination polished cross-section.

(6) A Zn—Al—Mg—Si alloy-plated steel material with excellent corrosionresistance according to (1), (3) or (4) above, characterized in that thetotal content of the bulky and scaly Mg₂Si phases in the plating layeris 10-30% as the area ratio when observed with a 5° inclination polishedcross-section, and the area ratio of bulky Mg₂Si to the total Mg₂Siphase is at least 1%.

(7) A Zn—Al—Mg—Si alloy-plated steel material with excellent corrosionresistance according to (2), (3) or (5) above, characterized in that thecontent of the scaly Mg₂Si phase in the plating layer is at least 3% asthe area ratio when observed with a 5° inclination polishedcross-section.

(8) A Zn—Al—Mg—Si alloy-plated steel material with excellent corrosionresistance according to any one of (1) to (7) above, characterized byhaving a preplating layer containing one or more from among Ni, Co, Zn,Sn, Fe and Cu and/or an intermetallic compound phase comprising two ormore from among Ni, Co, Zn, Sn, Fe and Cu, at the interface between theplating layer and the steel material.

(9) A Zn—Al—Mg—Si alloy-plated steel material with excellent corrosionresistance according to any one of (1) to (8) above, characterized inthat the plating coverage per side is 20-130 g/m².

(10) A process for production of a Zn—Al—Mg—Si alloy-plated steelmaterial with excellent corrosion resistance, which is a process forproduction of the Zn—Al—Mg—Si alloy-plated steel material according to(1) to (9) above characterized by keeping the temperature of the platingbath at 500-650° C. and controlling the cooling rate after plating to10° C./sec or greater.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of the 5° inclination polished cross-sectionalstructure of a plated steel sheet with a bulky Mg₂Si phase in theplating layer according to the present invention.

FIG. 2 shows an example of the 5° inclination polished cross-sectionalstructure of a plated steel sheet with a scaly Mg₂Si phase in theplating layer according to the present invention.

FIG. 3 shows an example of the perpendicular polished cross-sectionalstructure of a plated steel sheet with a bulky Mg₂Si phase in theplating layer according to the present invention.

FIG. 4 shows an example of the perpendicular polished cross-sectionalstructure of a plated steel sheet with a scaly Mg₂Si phase in theplating layer according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The Al—Zn—Mg—Si based plating layer according to the invention ischaracterized by having a specific alloy structure, but first the basicplating composition of the plated steel sheet will be explained. The Mgin the plating phase provides an effect of improving the corrosionresistance of the plated steel material. Addition of Mg at 0.5% orgreater (Throughout the present specification, the percentages given foraddition of elements in the alloy composition will be in terms of wt %unless otherwise specified.) provides an effect of improved corrosionresistance in saline environments, but in order to exhibit stablecorrosion resistance and effectively prevent edge creep after paintingeven in environments which are exposed to the outside atmosphere,addition of 1% or greater is necessary.

Although corrosion resistance is improved with increasing Mg addition,the corrosion resistance improving effect is saturated with addition ofMg in excess of 5% if the Si content of the plating layer is less than3%. The reason for this is thought to be that when the Mg content isless than 5% the added Mg is deposited as a scaly Mg₂Si phase, but whenthe Mg content exceeds 5% it is deposited as a Mg₂Zn or Mg₂Zn₃₁, phase.

On the other hand, if the Si content of the plating layer is 3% or more,an Mg addition of less than 3% will not be expected to exhibit acorrosion inhibiting effect due to the presence of a free Si monophase.Deposition of a bulky Mg₂Si phase begins when the Mg addition is 3% orgreater, and further increase in the addition of Mg improves thecorrosion resistance. However, when the amount of Mg added is increasedstill further, the viscosity of the bath gradually rises, impairing themanageability. If the amount of Mg added exceeds 10%, the depositedbulky Mg₂Si phase increases too much while the thickness of the poorlyworkable Fe—Al alloy layer at the iron substrate interface alsoincreases to the point of notably impairing the workability, resultingin reduced corrosion resistance.

In consideration of these factors, the preferred amount of Mg additionis at least 1% and less than 5% when the Si content is less than 3%, andat least 3% and less than 10% when the Si content is 3% or greater.

As regards the Si in the plating phase, if added in an amount of lessthan 0.5% a thick Fe—Al alloy layer is produced at the interface betweenthe iron substrate and the plating phase and plating cracks are inducedduring working, thus making it impossible to achieve sufficientworkability. This phenomenon occurs regardless of the amount of Mgadded, and therefore the amount of Si added must be at least 0.5%.

If Si is added at 3% or greater when the Mg addition is less than 3%, afree Si phase is deposited, thus impairing the workability andsignificantly reducing the corrosion resistance. On the other hand, whenthe Mg addition is 3% or greater, increasing addition of Si results ingreater deposition of the bulky Mg₂Si phase and improved corrosionresistance. However, addition of Si at 10% or greater drasticallyreduces the corrosion resistance.

For these reasons, two appropriate ranges exist for addition of Mg andSi, one being a range in which Si is at least 0.5% and less than 3% andMg is at least 1% and less than 5%, as the range in which a scaly Mg₂Siphase is deposited. The other is a range in which Si is at least 3% andless than 10% and Mg is at least 3% and less than 10%, as the range inwhich scaly and bulky Mg₂Si phases are deposited.

Persistent research by the present inventors on the Al/Zn ratio of theplating layer has revealed that the corrosion resistance-improvingeffect of the Mg₂Si phase is more notable with a higher Al/Zn ratio.When the Al/Zn ratio is less than 0.89, the corrosion resistance doesnot reach that of the Zn—Al plated steel sheet containing 25-75% Alproposed in Japanese Patent No. 617,971 even if a Mg₂Si phase isdeposited. When the Al/Zn ratio is over 2.75, the plating bathtemperature increases and hinders operation. From these considerations,the Al/Zn ratio of the plating layer was determined to be 0.89-2.75.

Turning now to the metal structure of the plating layer, FIG. 1 and FIG.2 schematically illustrate the structure of a plating layer according tothe present invention, as observed after polishing the plating layer ata 5° inclination. FIG. 1 shows an embodiment of the present inventionwhere the Al-rich dendritic phase 1 shown in white is a phase which hasgrown in a dendritic fashion, and it actually contains small amounts Zn,Mg, Si and Fe in solid solution. The Zn-rich dendritic phase 2 shown asthe dotted regions is also a phase which has grown in a dendriticfashion, and it actually contains small amounts of Al, Mg, Si and Fe insolid solution. The bulky Mg₂Si phase 3 is a deposited phase which hasbeen deposited as polygonal shapes with sizes of about a few tens ofmicrometers, and this phase is produced during the initial process ofplating aggregation. There are also dispersed and deposited MgZn₂ orMg₂Zn₁₁ structures as Zn—Mg based intermetallic compounds denoted byreference numeral 4 and having shapes which fill the gaps between thesephases, and a scaly Mg₂Si phase denoted by reference numeral 5.

FIG. 2 is another embodiment of the present invention and it differsfrom FIG. 1 only in that the bulky Mg₂Si phase 3 is not present.

On the other hand, FIG. 3 and FIG. 4 shows the results of observing thestructure after polishing the same sample perpendicular to its surface.The deposited phases corresponding to numerals in the drawings are thesame as in FIGS. 1 and 2. Reference numeral 6 is an Fe—Al based alloylayer, and reference numeral 7 is the steel substrate. In FIG. 3 where abulky Mg₂Si phase is deposited, the size is smaller than in FIG. 1 asobserved after polishing at a 5° inclination with respect to thehorizontal direction, and only the local form can be seen. This isbecause even though the bulky Mg₂Si phase is deposited in the state ofpolygonal plates spreading in the horizontal direction of the plating asthe initial solidified phase, only a very small portion thereof can beobserved when cutting is in the perpendicular direction by perpendicularpolishing. In some cases, the size that can be confirmed with 5°inclination polishing reaches 10 or more times the size that can beconfirmed with perpendicular polishing. Similarly, the Mg₂Si phasedeposited in a scaly form also differs considerably in the observablesize depending on the polishing angle. This is because the scaly Mg₂Siphase is deposited in a non-continuous manner in the gaps between theAl— and Zn— rich dendritic phases deposited in a dendritic fashion asthe primary crystals.

Thus, in order to accurately determine the shape and size of thedeposits, it is necessary to carry out polishing at an angle as close aspossible to the horizontal to the plating surface, and it is animportant aspect of the present invention that it was ascertained thatthe plating properties can be determined based on the size of the Mg₂Siphase determined accurately in this manner.

As a result of much research on the polishing angle by the presentinventors it was found that if an angle of 5° is maintained with respectto the horizontal direction, the size of the deposits that can beconfirmed is roughly the same as by horizontal polishing, and that thesize can be confirmed continuously from the plating surface layer to thebase iron section.

The forms and shapes of the Mg₂Si phase measured by this method will bedescribed below.

The bulky Mg₂Si phase is characterized in that the ratio of the shortdiameter with respect to the long diameter is 0.4 or greater, while thescaly Mg₂Si phase is characterized in that the ratio of the shortdiameter with respect to the long diameter is less than 0.4.

When the amounts of Mg and Si addition are low, the Mg₂Si phase isdeposited in a scaly form. When the amounts of Mg and Si addition exceed3%, deposition of a bulky Mg₂Si phase is simultaneously produced.Deposition of a bulky Mg₂Si phase is more satisfactory from thestandpoint of corrosion resistance, but in this case the characteristicspangle of the Zn—Al based plating will be lost. Selection may be madedepending on the need for spangle and the level of corrosion resistancerequired.

Regarding the size of the bulky Mg₂Si phase, if the average value forthe long diameter exceeds 50 μm, the particles act as origins forcracking, thus lowering the workability. Particularly, deposition ofparticles in excess of 100 μm induces peeling of the plating, and it istherefore necessary for the proportion of particles exceeding 100 μm inthe deposited bulky Mg₂Si phase to be controlled to no greater than 10%.Regarding the scaly Mg₂Si phase as well, the average value for the longdiameter must be controlled to no greater than 50 μm in order to ensureproper workability. The scaly Mg₂Si phase will not induce peeling of theplating even if particles exceeding 100 μm are deposited, but sufficientworkability can be ensured so long as the average value is controlled tono greater than 50 μm.

The size of the deposited Mg₂Si phase is affected most predominantly bythe cooling rate after hot-dip plating, and guaranteeing a cooling rateof at least 10° C./sec will allow the average value of the long diameterof either the bulky form or scaly form to be controlled to no greaterthan 50 μm. The cooling rate can be increased by controlling thecoverage with a wiping nozzle after plating, and then accomplishingcooling by forced blowing of air or an inert gas such as nitrogen. Watermist may also be blown in if it is desired to further increase thecooling rate. The lower limit for the size of the Mg₂Si phase is notparticularly restricted, but for normal operation with production at amaximum cooling rate of 50° C./sec, deposition of a size of about a fewμm is most common, and therefore 3 μm was established as the lowerlimit.

In order to sufficiently improve the corrosion resistance, the scalyMg₂Si phase content must be at least 3% in terms of area ratio asobserved with 5° inclination polishing. Deposition of a bulky Mg₂Siphase further improves the corrosion resistance, and particularly it isimportant for the proportion of the bulky Mg₂Si phase to be greater than1% with respect to the total Mg₂Si phase. On the other hand, if thetotal area ratio of the scaly Mg₂si phase and bulky Mg₂Si phase exceeds30% the workability is notably impaired, and therefore the upper limitis 30%.

The Zn—Al—Mg—Si alloy plating according to the invention ischaracterized by comprising one or more from among In: 0.01-1.0%, Sn:0.1-10.0%, Ca: 0.01-0.5%, Be: 0.01-0.2%, Ti: 0.01-0.2%, Cu: 0.1-1.0%,Ni: 0.01-0.2%, Co: 0.01-0.3%, Cr: 0.01-0.2%, Mn: 0.01-0.5%, Fe:0.01-3.0% and Sr: 0.01-0.5%. The purpose of adding one or more elementsfrom among In, Sn, Ca, Be, Ti, Cu, Ni, CO, Cr, Mn, Fe and Sr is tofurther improve the plating corrosion resistance, as it is believed thataddition of these elements further promotes passivation of the filmproduced on the plating surface. The effect of improving the corrosionresistance is exhibited when In, Sn, Ca, Be, Ti, Cu, Ni, Co, Cr, Mn, Feand Sr are added to at least 0.01, 0.1, 0.01, 0.01, 0.01, 0.1, 0.01,0.01, 0.01, 0.01, 0.01 and 0.01 wt %, respectively. On the other hand,if the addition amounts are too great a rough appearance is producedafter plating, with generation of outer appearance defects due to, forexample, dross, oxide adhesion and the like, and therefore the upperlimits for addition of each of the elements In, Sn, Ca, Be, Ti, Cu, Ni,Co, Cr, Mn, Fe and Sr are 1.0, 10.0, 0.5, 0.2, 0.2, 1.0, 0.2, 0.3, 0.2,0.5, 3.0 and 0.5 wt %, respectively.

Preplating may be carried out as pretreatment for the plating, in whichcase a preplating phase comprising one or more from among Ni, Co, Zn,Sn, Fe and Cu will be produced at the interface between the platinglayer and the base iron. An intermetallic compound phase may also formby reaction of the preplating layer and the base iron and plating metal.A mixed phase of the preplating phase and an intermetallic compoundphase may also result, but any of these situations are acceptable asthey do not hinder the gist of the invention. Dissolution or dispersionof the preplating in the plating bath can result in the preplatingcomponents being present in the plating layer, but this does not hinderthe gist of the invention. In particular, when this plating is appliedfor hot-rolled steel sheets or the like for the purpose of improvingplating adhesion, it is effective to carry out preplating with Ni atabout 0.5-1 g/m².

The plating coverage is preferably about 20-130 g/m² per side. Generallyspeaking, an increase in plating coverage is advantageous for thecorrosion resistance, and disadvantageous for the workability andweldability. The preferred coverage will therefore differ depending onthe purpose of use, but the coverage is preferably less for automobileparts which require excellent workability and weldability, and thecoverage is preferably more for building materials and electrichousehold appliances for which workability and weldability are not majorrequirements.

A post-treatment film such as a chemical treatment film or resin filmmay also be applied to the uppermost surface of the plating layer. Thiscan provide an improving effect on the weldability, coating adhesion,corrosion resistance, etc. A chemical treatment film or resin film maycontain one or more from among Si, C and P. Possible films includechromic acid-silica films, silica-phosphoric acid based films andsilica-resin based films, employing such widely used resin types asacrylic, melamine, polyethylene, polyester, fluorine, alkyd,silicone-polyester and urethane based resins. The film thickness is notparticularly restricted, and the treatment may usually be to about0.5-20 μm. Post-treatment may, of course, be applied as chromatingtreatment or treatment with an inhibitor solution containing nochromium.

The steel components of the parent material will now be explained. Noparticular restrictions are placed on the steel components, and theeffect of improvement in corrosion resistance is achieved for any typeof steel. The steel type may be IF steel, Al-k steel, Cr-containingsteel, stainless steel, high tension steel or the like, with addition ofTi, Nb, B, etc. Al-k steel or stainless steel is preferred forconstruction material purposes, Ti-IF steel is preferred for exhaustpipe purposes, Al-k steel is preferred for electrical appliancepurposes, and B-added IF steel is preferred for fuel tank purposes.

The plating bath temperature should not be below 500° C. to avoidraising the viscosity of the plating solution and thus hinderingoperation. On the other hand, a temperature exceeding 650° C. increasesthe alloy layer thickness produced at the steel/plating interface, thusimpairing the workability and corrosion resistance while also promotingdissolution loss of the plating equipment.

EXAMPLES Example 1 and Comparative Example 1

A cold-rolled steel sheet (sheet thickness: 0.8 mm) subjected toordinary hot rolling and cold rolling was used as the material forhot-dip Zn—Al—Mg—Si plating. The plating was accomplished using anon-oxidizing furnace/reducing furnace type line, and plating coverageadjustment by gas wiping after plating was followed by cooling and zerospangle treatment. The composition of the plating bath was varied toproduce test materials, and their properties were investigated. Fe waspresent in the bath at about 1-2% as an unavoidable impurity suppliedfrom the plating machine and strips in the bath. The bath temperaturewas 600-650° C. The obtained plated steel sheet was provided forstripping and plating composition and coverage measurement by chemicalanalysis methods, and the plating structure was observed with an opticalmicroscope after 5° inclination polishing. The corrosion resistance,workability, and weldability were simultaneously evaluated by thefollowing methods. The results are shown in Table 1.

(1) Corrosion Resistance Evaluation

i) Salt Corrosion Resistance

A test sample with dimensions of 70×150 mm was subjected to a salt spraytest according to JIS Z2371 for 30 days, and after stripping off thecorrosion product, the corrosion loss was measured. The corrosion lossvalues shown are for one plated side.

Evaluation Scale

⊚: Corrosion loss of ≦5 g/m²

∘: Corrosion loss of <10 g/m²

Δ: Corrosion loss of 10-25 g/m²

X: Corrosion loss of >25 g/m²

ii) Painted Corrosion Resistance

First, one side was subjected to chromic acid-silica based treatment to20 mg/m² based on metallic Cr, as chemical treatment. Next, a testsample with dimensions of 70×150 mm was subjected to 20 μmmelamine-based black painting, and baked at 140° C. for 20 minutes. Acrosscut was then formed and the sample was provided for a salt spraytest. The outer appearance after 60 days was visually observed.

Evaluation Scale

⊚: No red rust

∘: No red rust outside of crosscut

Δ: Red rust ratio ≦5%

X: Red rust ratio >5%

iii) Outdoor Exposure Test

The sample was painted after the chemical treatment described in ii)above. The painting was carried out with two types of paints, apolyethylene wax-containing acrylic-based resin (clear: 5 μm) and anepoxy-based resin (20 μm). After shearing to dimensions of 50×200 mm,the sample was subjected to an outdoor exposure test. The red rust ratioand surface coloration condition were observed from the edge after aperiod of 3 months.

Evaluation Scale

⊚: Red rust ratio from edge <30%

ΔA: Red rust ratio from edge 30-80%

X: Red rust ratio from edge >80%

(2) Weldability

After the chemical treatment described in ii) above, spot welding wasconducted under the welding conditions shown below, and the number ofcontinuous spots until the nugget diameter reduced to below 4t (t: sheetthickness) was evaluated.

Welding Conditions

Welding current: 10 kA, Pressure force: 220 kg, welding time: 12 cycles,Electrode diameter: 6 mm, Electrode shape: dome-shape, Tip: 6φ-40R

Evaluation Scale

⊚: Number of continuous spots >700

Δ: Number of continuous spots 400-700

∘: Number of continuous spots <400

(3) Workability

A cylindrical punch with a 50 mm diameter was used in a hydraulicmolding tester for cup molding at a draw ratio of 2.25. The test wascarried out with application of oil, and the flattening force was 500kg. The workability was evaluated on the following scale.

Evaluation Scale

∘: No defects

Δ: Cracks in plating

X: Peeling of plating

TABLE 1 Mg₂Si Proportion Bulky Long with Long Mg₂Si diameter longPlating components (%) Coverage diameter Volume proportion averagediameter Al Zn Mg Si Fe Al/Zn (g/m²) Form (μm) (%) (%) (μm) >100μPresent Invention 1 46 45.4 4 3.5 1.1 1.01 30 scaly + bulky 40 10.5 1840 2 2 48 35.8 7 8 1.2 1.34 40 scaly + bulky 35 22.5 60 35 0 3 50 28.59.5 9.3 1.5 1.75 35 scaly + bulky 30 29.5 71 30 0 4 55 36.8 3.5 3.5 1.21.49 50 scaly + bulky 32 10 9 32 0 5 55 31.7 7.5 4.8 1 1.74 70 scaly +bulky 45 15.1 41 45 5 6 58 26.9 9 5 1.1 2.16 50 scaly + bulky 42 21.2 5642 1 7 62 28 4 5 1 2.21 65 scaly + bulky 42 13.5 33 42 2 8 63 26.6 4.5 50.9 2.37 50 scaly + bulky 25 14.4 38 25 0 9 68 25 3 3 1 2.72 55 scaly +bulky 23 10 10 23 0 10 46 50.9 1.5 0.6 1 0.90 50 scaly 45 3 0 45 0 11 5044.9 3 1 1.1 1.11 45 scaly 46 6 0 46 0 12 58 36 3.5 1.5 1 1.61 55 scaly42 9 0 42 0 13 65 27.2 5 2 0.8 2.39 70 scaly 40 7.2 0 40 0 14 70 24.1 32 0.9 2.90 55 scaly 38 7.5 0 38 0 Comparative Examples 15 50 45.1 0.53.5 0.9 1.11 60 scaly 30 1.5 0 30 0 16 50 27 15 7 1 1.85 65 scaly +bulky 23 21 57 23 1 17 55 37.8 5 1 1.2 1.46 70 scaly 42 3.2 0 42 0 18 5522.5 6 15 1.5 2.44 70 scaly + bulky 23 20.1 55 23 0 19 50 33.5 8 7 1.51.49 35 scaly + bulky 75 23.2 61 75 15 20 50 33.5 8 7 1.5 1.49 10scaly + bulky 42 23.5 62 42 2 21 50 33.5 8 7 1.5 1.49 140 scaly + bulky42 22 59 42 1 22 25 58.5 8 7 1.5 0.43 70 scaly + bulky 42 24.2 63 42 223 55 41.6 0.1 2 1.3 1.32 40 none — — — — — 24 60 30.3 7 1.5 1.2 1.98 55scaly 38 3 0 38 0 25 58 37.5 3 0.2 1.3 1.55 40 none — — — — — 26 52 39.82 5 1.2 1.31 50 scaly 31 6 0 31 0 27 56 38.7 3 1 1.3 1.45 45 scaly 853.2 0 85 5 28 55 38.5 3 2 1.5 1.43 11 scaly 38 5.2 0 38 0 29 58 35.9 3 21.1 1.62 150 scaly 31 4.8 0 31 0 30 30 63.8 3 2 1.2 0.47 70 scaly 30 5 030 0 Corrosion Bath Cooling resistance temperature rate Salt Weld- Work-Overall (° C.) (° C./sec) corrosion Painting Exposure ability abilityevaluation Present Invention 1 630 25 ⊚ ⊚ ⊚ ∘ ∘ ⊚ 2 640 30 ⊚ ⊚ ⊚ ∘ ∘ ⊚ 3630 35 ⊚ ⊚ ⊚ ∘ ∘ ⊚ 4 630 30 ⊚ ⊚ ⊚ ∘ ∘ ⊚ 5 640 20 ⊚ ⊚ ⊚ ∘ ∘ ⊚ 6 630 25 ⊚⊚ ⊚ ∘ ∘ ⊚ 7 640 25 ⊚ ⊚ ⊚ ∘ ∘ ⊚ 8 630 40 ⊚ ⊚ ⊚ ∘ ∘ ⊚ 9 640 40 ⊚ ⊚ ⊚ ∘ ∘ ⊚10 630 15 ∘ ∘ ∘ ∘ ∘ ∘ 11 610 15 ∘ ∘ ∘ ∘ ∘ ∘ 12 620 20 ⊚ ⊚ ⊚ ∘ ∘ ⊚ 13 60020 ∘ ∘ ∘ ∘ ∘ ∘ 14 570 25 ∘ ∘ ∘ ∘ ∘ ∘ Comparative Examples 15 630 35 Δ ΔΔ ∘ ∘ Δ 1 Low Mg 16 640 40 ∘ ∘ ∘ ∘ x x 1 High Mg 17 620 25 Δ Δ Δ ∘ x x 1Low Si 18 640 40 x x x ∘ x x 1 High Si 19 630 5 ⊚ ⊚ ⊚ ∘ x x 1 Insuff.cool. rate 20 620 25 x x x ∘ ∘ x 1 Insuff. plat. cover. 21 620 25 ⊚ ⊚ ⊚x x x 1 High plat. cover. 22 620 25 Δ Δ Δ ∘ ∘ Δ 1 Low Al/Zn ratio 23 63030 Δ Δ Δ ∘ ∘ Δ 2 Low Mg 24 620 25 ∘ ∘ ∘ ∘ Δ Δ 2 High Mg 25 600 35 Δ Δ Δ∘ x x 2 Low Si 26 620 35 x x x ∘ x x 2 High Si 27 600 3 ∘ ∘ ∘ ∘ Δ Δ 2Insuff. cool. rate 28 590 25 x x x ∘ ∘ x 2 Insuff. plat. cover. 29 58030 ∘ ∘ ∘ x Δ x 2 High plat. cover. 30 600 35 Δ Δ Δ ∘ ∘ Δ 2 Low Al/Znratio

As comparative examples there are shown materials with slight additionof Mg (Sample Nos. 15 and 23), but both of these exhibited insufficientcorrosion resistance in the severe corrosion environments describedabove. With addition of excess amounts of Mg as with Sample Nos. 16 and24, the workability was impaired and the corrosion resistance wasconsequently insufficient. On the other hand, Sample Nos. 17 and 25which had insufficient amounts of Si addition had thicker alloy layersand exhibited inferior workability as well as insufficient corrosionresistance, while conversely, Sample Nos. 18 and 26 which had excessiveamounts of addition of Si exhibited inferior workability and corrosionresistance due to the effect of Si being deposited in the plating layer.

From the standpoint of the production conditions, Sample Nos. 19 and 27which were cooled at insufficient cooling rates after plating hadenlarged deposited Mg₂Si phases and inferior workability. Sample Nos. 20and 28 which had inadequate plating coverage exhibited insufficientcorrosion resistance, while Sample Nos. 21 and 29 which had excessivecoverage exhibited inadequate workability and weldability.

Sample Nos. 22 and 30 which had low Al/Zn ratios did not exhibit anadequate effect by the Mg₂Si phase, and the resulting corrosionresistance was inferior.

On the other hand, the invention example as represented by all of SampleNos. 1-14 exhibited excellent properties for all of the evaluatedparameters. The important property of corrosion resistance wasparticularly satisfactory when Mg and Si were higher within theirappropriate ranges.

Examples 2 and Comparative Example 2

A cold-rolled steel sheet with a thickness of 0.8 mm was used as thematerial for hot-dip plating by immersion for 3 seconds in a Zn—Al—Mg—Sialloy plating bath at a bath temperature of 630° C. The plating coveragewas adjusted to 90 g/m² by gas wiping after plating, and then coolingwas effected at a rate of 30° C./sec.

The compositions of the plating layers of each of the obtainedZn—Al—Mg—Si based steel sheets were as shown in Tables 2 and 3. Thecorrosion resistance was also evaluated by the methods described below.The results are shown in Tables 2 and 3. The structures of theseplatings as observed after 5° inclination polishing, at least in thecase of Example 2 (Sample Nos. 31-43) as in Example 1, were structurescomprising a bulky and scaly Mg₂Si phase as defined according to theinvention.

(1) Corrosion Resistance Evaluation

i) Salt Corrosion Resistance

A test sample with dimensions of 70×150 mm was subjected to a salt spraytest according to JIS Z2371 for 30 days, and after stripping off thecorrosion product, the corrosion loss was measured. The corrosion lossvalues shown are for one plated side.

Evaluation scale

⊚: Corrosion loss of ≦5 g/m²

∘: Corrosion loss of <10 g/m²

Δ: Corrosion loss of 10-25 g/m²

X: Corrosion loss of >25 g/m²

ii) Painted Corrosion Resistance

First, one side was subjected to chromic acid-silica based treatment to20 mg/m² based on metallic Cr, as chemical treatment. Next, a testsample with dimensions of 70×150 mm was subjected to 20 μmmelamine-based black painting, and baked at 140° C. for 20 minutes. Acrosscut was then formed and the sample was provided for a salt spraytest. The outer appearance after 60 days was visually observed.

Evaluation Scale

⊚: No red rust

∘: No red rust outside of crosscut

Δ: Red rust ratio ≦5%

X: Red rust ratio >5%

TABLE 2 Corrosion resistance Hot-dip Zn—Al—Mg—Si plating layercomposition (wt %) Salt Paint Al Mg Si In Sn Ca Be Ti Cu Ni Co Cr Mn FeSr corrosion layer 31 55 5 5 0.5 0.1> 0.01> 0.01> 0.01> 0.1> 0.01> 0.01>0.01> 0.01> 0.01> 0.01> ⊚ ⊚ Inv. 32 55 5 5 0.01> 2 0.01> 0.01> 0.01>0.1> 0.01> 0.01> 0.01> 0.01> 0.01> 0.01> ⊚ ⊚ Exs. 33 55 5 5 0.01> 0.1>0.1 0.01> 0.01> 0.1> 0.01> 0.01> 0.01> 0.01> 0.01> 0.01> ⊚ ⊚ 34 55 5 50.01> 0.1> 0.01> 0.05 0.01> 0.1> 0.01> 0.01> 0.01> 0.01> 0.01> 0.01> ⊚ ⊚35 55 5 5 0.01> 0.1> 0.01> 0.01> 0.1 0.1> 0.01> 0.01> 0.01> 0.01> 0.01>0.01> ⊚ ⊚ 36 55 5 5 0.01> 0.1> 0.01> 0.01> 0.01> 0.3 0.01> 0.01> 0.01>0.01> 0.01> 0.01> ⊚ ⊚ 37 55 5 5 0.01> 0.1> 0.01> 0.01> 0.01> 0.1> 0.050.01> 0.01> 0.01> 0.01> 0.01> ⊚ ⊚ 38 55 5 5 0.01> 0.1> 0.01> 0.01> 0.01>0.1> 0.01> 0.1 0.01> 0.01> 0.01> 0.01> ⊚ ⊚ 39 55 5 5 0.01> 0.1> 0.01>0.01> 0.01> 0.1> 0.01> 0.01> 0.05 0.01> 0.01> 0.01> ⊚ ⊚ 40 55 5 5 0.01>0.1> 0.01> 0.01> 0.01> 0.1> 0.01> 0.01> 0.01> 0.2 0.01> 0.01> ⊚ ⊚ 41 555 5 0.01> 0.1> 0.01> 0.01> 0.01> 0.1> 0.01> 0.01> 0.01> 0.01> 1.1 0.01>⊚ ⊚ 42 55 5 5 0.01> 0.1> 0.01> 0.01> 0.01> 0.1> 0.01> 0.01> 0.01> 0.01>0.01> 0.1 ⊚ ⊚ 43 55 5 5 0.01> 0.1> 0.01> 0.01> 0.01> 0.1> 0.01> 0.01>0.01> 0.01> 1.1 0.01> ⊚ ⊚ 44 55 5 5 0.01> 1 0.01> 0.01> 0.01> 0.1> 0.01>0.01> 0.01> 0.01> 1.1 0.01> ⊚ ⊚ 45 55 5 5 0.01> 0.1> 0.2 0.01> 0.01>0.1> 0.01> 0.01> 0.01> 0.01> 1.1 0.01> ⊚ ⊚ 46 55 5 5 0.01> 0.1> 0.01>0.1 0.01> 0.1> 0.01> 0.01> 0.01> 0.01> 1.1 0.01> ⊚ ⊚ 47 55 5 5 0.01>0.1> 0.01> 0.01> 0.05 0.1> 0.01> 0.01> 0.01> 0.01> 1.1 0.01> ⊚ ⊚ 48 55 55 0.01> 0.1> 0.01> 0.01> 0.01> 0.5 0.01> 0.01> 0.01> 0.01> 1.1 0.01> ⊚ ⊚49 55 5 5 0.01> 0.1> 0.01> 0.01> 0.01> 0.1> 0.1 0.01> 0.01> 0.01> 1.10.01> ⊚ ⊚ 50 55 5 5 0.01> 0.1> 0.01> 0.01> 0.01> 0.1> 0.01> 0.1 0.01>0.01> 1.1 0.01> ⊚ ⊚ 51 55 5 5 0.01> 0.1> 0.01> 0.01> 0.01> 0.1> 0.01>0.01> 0.1 0.01> 1.1 0.01> ⊚ ⊚ 52 55 5 5 0.01> 0.1> 0.01> 0.01> 0.01>0.1> 0.01> 0.01> 0.01> 0.3 1.1 0.01> ⊚ ⊚ 53 55 5 5 0.01> 0.1> 0.01>0.01> 0.01> 0.1> 0.01> 0.01> 0.01> 0.01> 1.1 0.3 ⊚ ⊚

TABLE 3 Corrosion resistance Hot-dip Zn—Al—Mg—Si plating layercomposition (wt %) Salt Paint Al Mg Si In Sn Ca Be Ti Cu Ni Co Cr Mn FeSr corrosion layer 54 55 5 5 1.2 0.1> 0.01> 0.01> 0.01> 0.1> 0.01> 0.01>0.01> 0.01> 0.01> 0.01> Δ Δ Comp. 55 55 5 5 0.01> 15 0.01> 0.01> 0.01>0.1> 0.01> 0.01> 0.01> 0.01> 0.01> 0.01> Δ Δ Ex. 56 55 5 5 0.01> 0.1>0.8 0.01> 0.01> 0.1> 0.01> 0.01> 0.01> 0.01> 0.01> 0.01> Δ Δ 57 55 5 50.01> 0.1> 0.01> 0.25 0.01> 0.1> 0.01> 0.01> 0.01> 0.01> 0.01> 0.01> Δ Δ58 55 5 5 0.01> 0.1> 0.01> 0.01> 0.23 0.1> 0.01> 0.01> 0.01> 0.01> 0.01>0.01> Δ Δ 59 55 5 5 0.01> 0.1> 0.01> 0.01> 0.01> 1.1 0.01> 0.01> 0.01>0.01> 0.01> 0.01> Δ Δ 60 55 5 5 0.01> 0.1> 0.01> 0.01> 0.01> 0.1> 0.220.01> 0.01> 0.01> 0.01> 0.01> Δ Δ 61 55 5 5 0.01> 0.1> 0.01> 0.01> 0.01>0.1> 0.01> 0.34 0.01> 0.01> 0.01> 0.01> Δ Δ 62 55 5 5 0.01> 0.1> 0.01>0.01> 0.01> 0.1> 0.01> 0.01> 0.21 0.01> 0.01> 0.01> Δ Δ 63 55 5 5 0.01>0.1> 0.01> 0.01> 0.01> 0.1> 0.01> 0.01> 0.01> 0.52 0.01> 0.01> Δ Δ 64 555 5 0.01> 0.1> 0.01> 0.01> 0.01> 0.1> 0.01> 0.01> 0.01> 0.01> 3.2 0.01>Δ Δ 65 55 5 5 0.01> 0.1> 0.01> 0.01> 0.01> 0.1> 0.01> 0.01> 0.01> 0.01>0.01> 0.52 Δ Δ

Industrial Applicability

The present invention provides surface-treated steel sheets with highcorrosion resistance of the plating layers as well as highlysatisfactory edge creep resistance after painting. Their use may beapplied for virtually all conventional surface-treated steel sheets, andthe contribution to industry is therefore highly significant.

What is claimed is:
 1. A Zn—Al—Mg—Si alloy-plated steel material withexcellent corrosion resistance, characterized by comprising, in terms ofwt %, Al: at least 45% and no greater than 70% Mg: at least 3% and lessthan 10% Si: at least 3% and less than 10%, with the remainder Zn andunavoidable impurities, wherein the Al/Zn ratio is 0.89-2.75 and theplating layer contains a bulky Mg₂Si phase.
 2. A Zn—Al—Mg—Sialloy-plated steel material with excellent corrosion resistance,characterized by comprising, in terms of wt %, Al: at least 45% and nogreater than 70% Mg: at least 1% and less than 5% Si: at least 0.5% andless than 3%, with the remainder Zn and unavoidable impurities, whereinthe Al/Zn ratio is 0.89-2.75 and the plating layer contains a scalyMg₂Si phase.
 3. A Zn—Al—Mg—Si alloy-plated steel material with excellentcorrosion resistance, characterized in that the bulky Mg₂Si phase ofclaim 1 has a long diameter mean size of 3-50 μm, the area ratio ofparticles with a long diameter exceeding 100 μm is no more than 10% ofthe bulky Mg₂Si phase, and the ratio of the short diameter to the longdiameter is at least 0.4, as observed with a 5° inclination polishedcross-section.
 4. A Zn—Al—Mg—Si alloy-plated steel material withexcellent corrosion resistance, characterized in that the scaly Mg₂Siphase of claim 2 has a long diameter mean size of 3-50 μm. and the ratioof the short diameter to the long diameter is less than 0.4, as observedwith a 5° inclination polished cross-section.
 5. A Zn—Al—Mg—Sialloy-plated steel material with excellent corrosion resistanceaccording to claims characterized in that the total content of the bulkyand scaly Mg₂Si phases in the plating layer is 10-30% as the area ratiowhen observed with a 5° inclination polished cross-section, and the arearatio of bulky Mg₂Si to the total Mg₂Si is at least 1%.
 6. A Zn—Al—Mg—Sialloy-plated steel material with excellent corrosion resistanceaccording to claim 4 characterized in that the content of the scalyMg₂Si phase in the plating layer is at least 3% as the area ratio whenobserved with a 5° inclination polished cross-section.
 7. A Zn—Al—Mg—Sialloy-plated steel material with excellent corrosion resistanceaccording to claim 1, characterized by further comprising, as theZn—Al—Mg—Si alloy plating composition, one or more from among In:0.01-1.0%, Sn: 0.1-10.0%, Ca: 0.01-0.5%, Be: 0.01-0.2%, Ti: 0.01-0.2%,Cu: 0.1-1.0%, Ni: 0.01-0.2%, Co: 0.01-0.3%, Cr: 0.01-0.2%, Mn:0.01-0.5%, Fe, 0.01-3.0% and Sr: 0.01-0.5%.
 8. A Zn—Al—Mg—Sialloy-plated steel material with excellent corrosion resistanceaccording to claim 2, characterized by further comprising, as theZn—Al—Mg—Si alloy plating composition, one or more from among In:0.01-1.0%, Sn: 0.1-10.0%, Ca: 0.01-0.5%, Be: 0.01-0.2%, Ti: 0.01-0.2%,Cu: 0.1-1.0%, Ni: 0.01-0.2%, Co: 0.01-0.3%, Cr: 0.01-0.2%, Mn:0.0.01-0.5%, Fe: 0.01-3.0% and Sr: 0.01-0.5%.
 9. A Zn—Al—Mg—Sialloy-plated steel material with excellent corrosion resistanceaccording to claim 1, characterized in that the total content of thebulky and scaly Mg₂Si phases in the plating layer is 10-30% as the arearatio when observed with a 5° inclination polished cross-section, andthe area ratio of bulky Mg₂Si to the total Mg₂Si phase is at least 1%.10. A Zn—Al—Mg—Si alloy-plated steel material with excellent corrosionresistance according to claim 2, characterized in that the content ofthe scaly Mg₂Si phase in the plating layer is at least 3% as the arearatio when observed with a 5% inclination polished cross-section.
 11. AZn—Al—Mg—Si alloy-plated steel material with excellent corrosionresistance according to claim 1, characterized by having a preplatinglayer containing one or more from among Ni, Co, Zn, Sn, Fe and Cu and/orthe intermetallic compound phase comprising two or more from among Ni,Co, Zn, Sn, Fe and Cu, at the interface between the plating layer andthe steel material.
 12. A Zn—Al—Mg—Si alloy-plated steel material withexcellent corrosion resistance according to claim 2, characterized byhaving a preplating layer containing one or more from among Ni, Co, Zn,Sn, Fe and Cu and/or the intermetallic compound phase comprising two ormore from among Ni, Co, Zn, Sn, Fe and Cu, at the interface between theplating layer and the steel material.
 13. A Zn—Al—Mg—Si alloy-platedsteel material with excellent corrosion resistance according to claim 1,characterized in that the plating coverage per side is 20-130 g/m². 14.A Zn—Al—Mg—Si alloy-plated steel material with excellent corrosionresistance according to claim 2, characterized in that the platingcoverage per side is 20-130 g/m².